Combustion tailoring of solid propellants by oxidizer encasement

ABSTRACT

Solid rocket fuel particles such as aluminum, beryllium, magnesium, or boron are encased in an oxidizer by crystallizing the oxidizer onto the fuel particles. Encasement of the fuel particles before binding with a binder matrix enhances the combustion efficiency of the propellant.

United States Patent 1 1 3,706,608

Geisler 1 1 Dec. 19, 1972 [S4] COMBUSTION TAILORING OF SOLID [56]References Cited PROPELLANTS BY OXIDIZER ENCASEMENT UNITED STATESPATENTS [72] Inventor: Robert L. Geisler Lancaster Calif 2,982,6405/196] Blake ..l49/8 3,l20,459 2/1964 Coates et al ..l49/5 [73]Assignee: The United States of America as i represented by the secretaryof the Primary ExamznerCarl D. Quarforth Fol-ce AssistantExaminerStephen J. Lechert, .lr.

AttorneyHarry A. Herbert, Jr. and Cedric H. Kuhn [22] Filed: March 24,1970 21] Appl. No.: 24,894 [571' ABSTRACT Solid rocket fuel particlessuch as aluminum, berylli- 1521 vs. c1. ..l49/6, 149/5, 149/19,magnesium are encased in Miler 149/20, 149/22, 149/42, 149/43, 149/44 bycrystallizing the oxidizer onto the fuel particles. En- 49/ 0 149/ 1149/7 149/35 2 4 3 C casement of the fuel particles before binding witha 1511 Int. Cl. ..C06b 19/02 binder matrix enhances the Combustionefficiency of Field of Search ..149/5, 8, 6, 19, 20, 22, 42, the p p 3Claims, 2 Drawing Figures COMBUSTION TAILORING OF SOLID PROPELLANTS BYOXIDIZER ENCASEMENT BACKGROUND OF THE INVENTION 1. Field of theInvention This invention is in the field of solid rocket propellants.

2. Description of the Prior Art The mixing and casting of solid oxidizerparticles and solid fuel particles in hydrocarbon binders to form a 1solid propellant grain is well known. It is also well known that, inorder to burn efficiently, a solid propellant grain should combust in asmooth, layer-by-layer manner with combustion beginning at the surfaceof an opening running through its center and proceeding smoothlyoutwardly toward the case of the rocket motor.

In the prior art, oxidizer and fuel particles have been mixed into andcontained by the binder matrix as discrete entities. Studies have shownthat this results in propellant grains made up of relatively largeoxidizer particles surrounded by pockets of smaller fuel particleswithin the binder matrix. Upon combustion, pockets of fuel particlesexposed at the burning surface become rapidly heated, melt together, andexude as molten droplets onto the surface of the propellant. The moltendroplets then roll around on the surface until several of them cametogether and form an agglomerate. Agglomerates thus formed are generallyto 100 times larger than the original fuel particles found in thepockets of unburned propellant grains. After formation, the agglomeratesare transported into the combustion chamber by high velocity gaseousdecomposition products formed by reaction between oxidizer and binder.During the process of agglomerate formation and transportation into thecombustion chamber, from which they exit through the rocket nozzle, thefuel particles act as reducing agents and become oxidized as a result ofreaction with oxidizer particles. The agglomerate formation process hasbeen found to hamper the efficiency of the oxidation-reduction process.Further, the melting action prior to the formation of agglomeratesleaves pits in the burning surface of the propellant grain whichinterfere with the smooth, layer-by-layer combustion necessary to obtainthe highest combustion efficiency.

SUMMARY OF THE INVENTION It has now been found that combustionefficiency of BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a crosssection of a typical prior art propellant grain during combustion; and

FIG. 2 shows a cross section of a propellant grain contemplated by thepresent invention during combustion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention may beconveniently understood by viewing a cross section of a typical priorart solid propellant grain in conjunction with a cross section of apropellant grain of the type contemplated herein. FIG. 1 of the drawingis a cross section of a typical prior art propellant grain during theprocess of combustion. Ox-

0 idizer particles 1 and fuel particles 2 are shown as discreteparticles within a binder matrix 3. During combustion, pockets ofdiscrete fuel particles melt together to form larger single particles 4.Then, varying numbers of larger, melted together particles 4 roll aroundon the grain surface and form into agglomerates 5. High velocity gasescreated by reaction between the oxidizer and the binder carry theagglomerates away from the surface of the propellant grain into thecombustion chamber proper where they are oxidized and blown out throughthe rocket nozzle. The agglomeration process lowers the efficiency ofoxidation by lowering the surface area of fuel material exposed to theheat of the combustion chamber and in contact with oxidizer. Also,smooth layer-by-layer burning of the propellant is hampered when pocketsof discrete fuel particles 2 melt together to form the larger singleparticles 4 leaving pits in the propellant grain surface.

In contrast to FIG. 1, FIG. 2 shows a propellant grain of this inventionwherein, prior to being incorporated into the binder 3, the fuelparticles 1 have been provided with a coat of oxidizer 2 bycrystallizing oxidizer material on them. Coating of the fuel particleswith ox idizer increases the efficiency of combustion of the fuelparticles by insuring that the oxidation-reduction reaction betweenoxidizer and fuel takes place while the fuel particle is in its small,original size. For a given weight of fuel particles in a propellantgrain, a greater amount of surface area is exposed for purposes ofreaction by keeping the fuel particles discrete from one another withoxidizer coats than is accomplished by the prior art method describedabove. Further, coating fuel particles with oxidizer prevents theformation of pockets caused by the melting together of pockets ofparticles prior to agglomerate formation. Thus, more smooth,layer-by-layer burning is accomplished. Efficiency gains of from 2 to 10percent may be obtained by coating the fuel particles as shown by FIG. 2because the fuel is oxidized prior to its entry into the combustionchamber proper rather than being allowed to agglomerate.

Burn rate catalysts may also be coated with oxidizer to assist in thetailoring of combustion. Either burn rate accelerators such as ironoxide, iron blue, and copper chromate or burn rate decelerators such asoxamides, barbiturates, and guanadine derivatives may be coated withoxidizer and utilized to tailor the combustion rate as desired. Sincecoating places the accelerators or decelerators, whichever the case maybe, in more intimate contact with the oxidizer, their action is mademore effective at a given concentration level.

Examples of fuel particles and oxidizers which may be utilized inconjunction with one another according to this invention include suchfuels as aluminum, beryllium, magnesium, and boron particles and suchoxidizers as ammonium perchlorate, ammonium nitrate,cyclotetramethylenetetranitramine (I-IMX), cyclotrimethylenetrinitramine(RDX), and metal perchlorates such as alkali metal perchlorates. Severalmethods are available for the crystallization of a solute onto a seedparticle suspended in solution.

EXAMPLE 1 Aluminum powder is placed in a saturated solution of ammoniumperchlorate in water. The ammonium perchlorate is then shocked out ofsolution by rapidly lowering the temperature until a precipitateappears. The precipitate is a plurality of aluminum particle nucleicoated with ammonium perchlorate.

EXAMPLE ll EXAMPLE Ill Ammonium perchlorate coated fuel particles areobtained bydripping a nonsolvent, ethyl ether, into saturated solutionsof ammonium perchlorate in the solvents of Examples I and II containingsuspended fuel particles. The coated particles are then centrifuged andthe liquid decanted.

EXAMPLE IV Saturated solutions of ammonium nitrate, various alkali metalperchlorates such as sodium perchlorate or potassium perchlorate, HMX,and RDX are used in lieu of the ammonium perchlorate solutions of theprevious examples and the methods of the previous examples utilized withsimilar results.

EXAMPLE V Surface treatment of aluminum powder may be used to enhancethe process of Example I. In this embodiment, aluminum powder is treatedwith perchloric acid to form a thin surface layer of the perchloratederivative on aluminum. The resulting coated aluminum is thensubstituted for the nontreated aluminum powder of Example I and themethod of Example I used to coat the treated aluminum.

Separation of oxidizer coated fuel particles from the saturatedsolutions of oxidizer may be accomplished either by centrifugal orsedimentation methods taking advantage of differences in specificgravity.

After preparing the oxidizer coated fuel particles of oxidizer coatedburn rate catalysts, the coated particles may be incorporated intoabinder by standard propellant processing techniques. The binder, coatedparticles (either fuel, catalyst, or both), and any other additives maybe charged into a standard horizontal or vertical propellant mixer andmixed at temperatures of from about F to about F. After mixing, thepropellant is cast into molds or rocket motor cases with suitablemandrels and cured. Typical compositions which may be prepared are:

Binder-l2 to 20% by weight oxidizer-60 to 75% by weight Metal Fuel-0020%b we t wherein the bintler may lie lig'liroxy or carboxy terminatedpolybutadiene, a copolymer of butadiene with acrylic acid and/oracrylonitrile. The propellant may be plasticized with liquid hydrocarbonto improve processibility and mechanical properties. Either all or onlypart of the fuel may be oxidizer coated. Curatives may be added to thebinder to achieve an infinite polymer network as is usual in the art.

I claim:

1. The method of tailoring solid rocket propellants comprising the stepsof:

a. coating solid metal fuel particles with a crystallized coat ofoxidizer selected from the group consisting of ammonium perchlorate,ammonium nitrate, metal perchlorates, cyclotetramethylenetetranitramineand cyclotrimethylenetrinitramine;

b. mixing the oxidizer coated fuel particles witha binder material; and

c. curing the binder.

2. The method of tailoring solid rocket propellants comprising the stepsof:

a. coating solid fuel particles selected from the group consisting ofaluminum, beryllium, magnesium and boron with a crystallized coat ofoxidizer selected from the group consisting of ammonium perchlorate,ammonium nitrate, metal perchlorates, cyclotetramethylenetetranitramineand cyclotrimethylentrinitramine;

b. mixing the oxidizer coated fuel particles with a binder material; and

c. curing the binder.

3. The method according to claim 2 wherein the binder is selected fromthe group consisting of carboxy terminated polybutadiene, hydroxyterminated polybutadiene, a copolymer of butadiene and acrylic acid, acopolymer of butadiene and acrylonitrile, and a terpolymer of butadienewith acrylic acid and acrylonitrile.

2. The method of tailoring solid rocket propellants comprising the stepsof: a. coating solid fuel particles selected from the group consistingof aluminum, beryllium, magnesium and boron with a crystallized coat ofoxidizer selected from the group consisting of ammonium perchlorate,ammonium nitrate, metal perchlorates, cyclotetramethylenetetranitramineand cyclotrimethylentrinitramine; b. mixing the oxidizer coated fuelparticles with a binder material; and c. curing the binder.
 3. Themethod according to claim 2 wherein the binder is selected from thegroup consisting of carboxy terminated polybutadiene, hydroxy terminatedpolybutadiene, a copolymer of butadiene and acrylic acid, a copolymer ofbutadiene and acrylonitrile, and a terpolymer of butadiene with acrylicacid and acrylonitrile.